1. Field of the Invention
This invention relates to heat-insulating plates and more particularly to heat-insulating plates which are used with vibrating heat sources, such as motor vehicles.
2. Prior Art
In the prior art, automobile engines are usually provided with heat-insulating plates around the exhaust manifolds and hot air intakes, etc. (which are the principle source of heat in motor vehicle engine) in order to alleviate heat damage from these heat sources. Conventionally, however, materials with a poor damping effect (e.g. steel plate) have been used for the heat-insulating plates. In order to enhance the heat-insulating effect, such heat-insulating plates are mounted on heat sources so that there is a slight gap between the heat-insulating plate and the surface of the heat source. As a result, such heat-insulating plates have suffered from the following drawbacks. In the case of vibrating heat source such as an automobile engine, etc., the heat-insulating plate itself undergoes elastic vibration so that it becomes an additional source of noise and generates an intense radiant sound. This drawback is especially severe for the case of heat-insulating plate which has a cantilever structure and which is mounted on the engine itself (such as heat-insulating plates which are used to heat-insulate the exhaust manifold of the motor vehicle engine).
To solve this problem, it would be possible to change the resonant frequency of the heat-insulating plate by altering the weight of the heat-insulating plate or installing weights on the plate in order to reduce the vibration of the heat-insulating plate accompanying the vibration of the engine. However, in the case of a vibrating heat source whose vibrating frequency varies greatly with conditions of operation, such as a motor vehicle engine, it is very difficult to set the resonant frequency of the heat-insulating plate to avoid the entire range of vibration frequencies generated. Furthermore, such a tactic, moreover, fails to provide a real solution to the problem.
In addition to solve this problem it would also be possible to make the entire heat-insulating plate of asbestos instead of metal plate. However, the problem arises here in that if asbestos is used for the portion of the heat-insulating plate to which a great thermal effect is exerted by the heat source, the heat will turn the asbestos into a powder. Accordingly, asbestos cannot be used in this way in heat-insulating plates.
The present Applicant has previously proposed a vibration-absorbing heat-insulating plate such as the one shown in FIGS. 1 and 2 as a means for eliminating the above drawbacks. Such vibration-absorbing heat-insulating plate is described in Application for U.S. Pat. No. 882,195. This heat-insulating plate is installed, for example, around the exhaust manifold 6 of a six-cylinder engine 4. The heat-insulating plate consists of a metal plate 10 which covers the surface of the area extending from the second cylinder to the sixth cylinder where the greatest thermal effect is exerted by the exhaust manifold 6. For the sake of illustration, that portion of the plate covering the area extending from the fifth cylinder to the sixth cylinder is omitted from the figures. In addition a vibration-absorbing part 12 which covers the surface of the area at the front end in the vicinity of the first cylinder, where the thermal effect is relatively small, is integrally connected by a joint 11 to the steel plate 10 and is principally made of asbestos. Bolts 14 fasten the vibration-absorbing heat-insulating plate 8 to the engine 4 along the manifold 6. By constructing a vibration-absorbing heat-insulating plate described above, it is possible to create a heat-insulating plate which suppresses vibration of the engine without sacrificing the heat-insulating effects of the heat-insulating plate. Accordingly, it is possible to suppress radiant noise generated by the heat-insulating plate. However, this device has suffered from a particular disadvantage. The disadvantage is that depending on the structure of the vibration-absorbing part, the vibration-absorbing part is difficult to manufacture and had poor durability.